1. Field of the Invention
The present invention relates to a power transfer apparatus having a vibration dampening mechanism, and more particularly, a power transfer apparatus disposed between a crankshaft of a power producing device, such as an internal combustion engine, and a power output device, such as an automotive transmission, for transmitting torque therebetween, where portions of the vibration dampening mechanism provide structural support for the power transfer apparatus in the presence of axial stresses, such as thrust force in an axial direction on the power transfer apparatus.
2. Description of Related Art
Power transfer apparatus having a pair of flywheels and a clutch mechanism, usually placed between a crankshaft of an engine and a transmission are well known.
For example, there are flywheel assemblies in which two flywheels are combined to absorb vibration from an engine. One such flywheel assembly includes a first flywheel connected to the crankshaft in the engine, a second flywheel supported by the first flywheel for limited relative rotation therebetween, and a dampening element elastically connecting the first flywheel to the second flywheel in a circumferential direction for dampening torsional vibration between the first and second flywheels. The second fly wheel has a friction surface formed on its one side close to a transmission, and a clutch is fixed to the friction surface.
The clutch is comprised mainly of a clutch disk assembly and a clutch cover assembly. The clutch disk assembly has an annular clutch disk which selectively contacts the friction surface of the second flywheel, and a hub flange having splines mated with a main drive shaft in the transmission. The clutch cover assembly includes a saucer-like clutch cover having its exterior circumferential edge fixed to the flywheel, an annular pressure plate enclosed in the clutch cover for pressing the clutch disk against the friction surface of the second flywheel, and a diaphragm spring held by the clutch cover for urging the pressure plate toward the second flywheel.
Disadvantages of the prior art power transfer device will be listed below.
1) The flywheel assembly and the clutch separately made, make for a large number of parts, and costly manufacturing.
2) In disengaging the clutch, the forces associated with the release load are transmitted to the second flywheel via the clutch cover. The force applied to the second flywheel is then transmitted to a bearing disposed between the first and second flywheels. The bearing must be able to withstand the forces associated with disengagement of the clutch. Therefore, the bearing must be able to withstand large forces and still be rotatable. However, such a bearing is expensive and occupies a large space in radial direction. This impose a great restriction upon the design of the interior of the dampening element in the flywheel mechanism.
3) If the first flywheel of the power transfer device is mounted to a disk-shaped flexible plate which is mounted to the crankshaft, and the flexible plate and the first flywheel are fixed at their respective outer circumferences to each other, and a bearing is provided surrounding a boss of the first flywheel to hold the second flywheel rotatably relative to the first flywheel, then it is difficult to ensure accurate concentric positioning of the flexible plate with the first flywheel, and the concentric positioning of other parts.
In another prior art flywheel assembly, a fluid duct filled with fluid is defined by a disk-shaped element within a chamber partially defined by the first flywheel. A dampening element for dampening torsional vibration is provided within the chamber. The first flywheel has a center boss extending toward the transmission, and the second flywheel is supported thereon via a bearing which encircles the center boss of the first flywheel allowing for limited relative rotation between the two flywheels.
The flywheel assembly may be provided with a flexible plate which can flex itself in an axial direction between the crankshaft in the engine and the first flywheel in order to absorb flexural vibration from the engine. The flexible plate has its inner circumferential edge fixed to tip of the crankshaft and its outer circumferential edge fixed to the outer circumference of the first flywheel. A plurality of bolts are disposed circularly at the same intervals. A ring gear to start the engine is fixed at the outer circumference of the first flywheel.
Some of the disadvantages of the flywheel assembly will be listed below:
1) A seal member must be provided between the disk element and a power output element to seal the fluid within the chamber. It is desirable that a pre-load is applied to the bearing supporting the first and second flywheels, and for that purpose, an elastic element must be provided. As will be recognized, provision of the seal element and the elastic element increases the number of parts, which in turn causes a cost to rise.
2) The dynamics of the flywheel assembly are such that flexural vibration from the engine may produce noise. The damper mechanism requires a certain level of inertia in order to absorb vibrations. If there is insufficient inertia, the flywheel assembly cannot absorb torsional vibration from the engine.
3) Since the flexible plate and the first flywheel are fixed at their respective circumferences to each other, relative location among the flexible plate and the boss and bearing for the first flywheel are determined at the outer circumferences of the flexible plate and the first flywheel, and this causes the parts to be less concentric. The bearing is positioned outside a pitch circle of the bolts, and this imposes a restriction upon the design of the interior of the damper.
4) A power transfer device system including a damper in an automobile must increase moment of inertia of a power output mechanism to reduce a resonance frequency to an engine's idling speed or below. However, the ring gear to start the engine is fixed to the outer circumference of the first flywheel, and hence, a ratio of the moment of inertia of the power output mechanism cannot be sufficiently increased.
In another prior art mechanism, a damper mechanism includes a hub flange, a bearing and a dampening element. A power input element is coupled to a crankshaft in the engine while the hub flange is coupled to a main drive shaft extending from the transmission. The hub flange has a boss extending toward the transmission and a flange formed at the outer circumference of the boss. The bearing is provided between a power receiving element and the hub flange to support both of them rotatably relative to each other. The dampening element is provided within a fluid space to elastically connect the power receiving element and the hub flange in a circular direction and further to dampen torsional vibration between them. A driven plate connecting the dampening element and the hub flange is provided therebetween. The dampening element includes an elastic element provided in a window spreading toward the circumference of the driven plate, and a resistance generating mechanism for producing resistance when the input receiving element and the driven plate are relatively rotated. The window in the driven plate supports the elastic element which expands or contracts due to the torsional vibration. When the window is small in thickness, a greater bearing pressure is applied to the window, and an edge of the window has its lifetime shortened. Thus, a plurality of disk-like sheet metal plates or thick casting parts may be used to thicken and reinforce the driven plate.
Disadvantages of the prior art damper are listed below:
1) Since the hub flange and the driven plate are separately made, there are a large number of parts making the overall configuration complicated and costly to manufacture. Moreover, the flange of the boss extends outward to support the bearing. The boss itself, as a whole, is bulky. Since the boss is made of casting material, it has considerable mass and weight and is costly to manufacture.
2) The boss in the power output element protrudes toward the transmission. Thus, the damper is large in an axial direction.
3) A seal element must be provided between the hub flange and other parts to seal the fluid space. The number of components increases because of the seal element making the device costly to manufacture.
4) The hub flange is supported by the bearing on an outer circumference of the boss in the power input element so as to rotate relative to the power input element. The bearing is affected by thrust load and radial load. Thus, the bearing employed herein must be sufficiently large in the radial direction. With such a large bearing, manufacturing is costly, and the bearing occupies a large space in the radial directions. Consequently, a restriction is imposed upon a design of the inside of the dampening element.
5) The driven plate has an undesirably large mass and weight increasing cost and adding weight to the machine it is mounted in. In the case an automobile, where weight reduces fuel efficiency, this is undesirable.